José Manuel Astilleros

Molecular-scale surface processes during the growth of calcite in the presence of manganese

This paper deals with the growth behaviour of the Mn-Ca-CO3-H2O solid solution-aqueous solution system on calcite {104} surfaces. This system represents a model example, which allows us to study the effect of a number of controlling factors on the crystallisation: (1) the supersaturation function, β(x), and nucleation rate function, J(x), for the Mn-Ca-CO3-H2O system, (2) the relationship of such functions to the molecular scale growth mechanisms operating on growing surfaces, and (3) the surface structure of the calcite {104} faces. In situ atomic force microscopy (AFM) growth experiments revealed a wide variety of surface phenomena, such as the transition between growth mechanisms, anisotropic changes in the step rates, and the influence of the Mn-bearing newly formed surface on subsequent growth (step stoppage followed by the formation of two-dimensional nuclei and the reproduction of the original calcite {104} surface microtopography). These phenomena result from the interplay between the controlling parameters and are explained in those terms.

The effect of barium on calcite {101¯4} surfaces during growth

In situ atomic force microscopy (AFM) experiments have provided information about the effect of Ba21 on crystal growth of calcite {101¯4} surfaces. The microtopographic features observed have been interpreted by considering both the structural control that the calcite surfaces exert on the incorporation of divalent cations and the supersaturation state of the solution used. Pinning of the calcite growth steps occurs at low Ba concentrations, suggesting specific sites for Ba incorporation. When the Ba content of the solution is increased the advancement of monomolecular steps is observed. Although [4¯41]1 and [481¯]1 steps advance showing characteristic jagged edges, the parallel steps (i.e., [4¯41]2 and [481¯]2) remain practically immobile. This fact can be explained by considering the nonsymmetrically related distribution of large and small sites along the calcite steps and the easier incorporation of barium on the former. The measured increase in the height of the newly grown steps is also consistent with such preferential incorporation of Ba in certain positions. A further increase in the Ba concentration of the solutions leads to the formation of bidimensional nuclei on the calcite {101¯4} surfaces. The nature of these nuclei is discussed taking into account the supersaturation of the solution with respect to two possible structures that can accommodate Ba: the calcite-type structure and the aragonite-type structure.

Metastable phenomena on calcite {101̄4} surfaces growing from Sr2+–Ca2+–CO32− aqueous solutions

In situ atomic force microscopy (AFM) experiments, scanning electron microscopy (SEM) imaging and composition analysis, and X-ray diffraction have provided information about the growth, dissolution and transformation processes promoted by Sr2 + –Ca2 + –CO3 2 aqueous solutions in contact with calcite {101¯4} surfaces. Experiments have shown a wide variety of surface phenomena, such as the influence of the Sr-bearing newly-formed surface on the subsequent growth (template effect), the growth and subsequent dissolution of surfaces and the nucleation of secondary three-dimensional nuclei on calcite surfaces. These phenomena reveal the metastability of the crystallisation system and are a consequence of the interplay between thermodynamics (the relative stability of the two calcite and aragonite structure solid solutions that can be formed), supersaturation of the aqueous solution with respect to the two possible solid solutions, and the crystallographic control of the surfaces on cation incorporation.

Supersaturation functions in binary solid solution–aqueous solution systems

In this paper, we present a brief review of the thermodynamic equilibrium of binary solid solution–aqueous solution (SS-AS) systems and derive an expression hat allows us to evaluate the supersaturation or undersaturation of a given aqueous solution with respect to the whole range of solid compositions: the δ(x) function. Such an expression is based on the two conditions that define the SS-AS thermodynamic equilibrium. The derivation of the new supersaturation function, δ(x), was made by considering in detail the compositional relationships between solid and aqueous phases. To represent the new formulation on Lippmann diagrams, we have defined a new thermodynamic concept: the “actual activity.” In addition, we show how our supersaturation function behaves for both ideal and subregular solid solutions. The behaviour and applicability of both the δ(x) function and a previous supersaturation function, β(x), defined by Prieto et al. (1993), is discussed.

In situ HAFM study of the thermal dehydration on gypsum (010) surfaces

Abstract Hydrothermal AFM has been used to study the thermal dehydration reaction on gypsum (010) surfaces in solutions at different saturation states, and in the absence of a bulk liquid phase. Experiments were carried out at temperatures ranging from 25 to 130 °C. Whereas supersaturated solutions (β = 1.8.5) caused gypsum growth in the entire temperature range, solutions close to equilibrium (β = 1.02) caused various responses of the gypsum surface. The most prominent was a sharp transition from fast growth to very fast dissolution at ~120 °C suggesting a sudden nucleation of a phase more stable than gypsum. No structural relation could be found between the parental gypsum (010) surface and the crystallizing phase. In the absence of a bulk liquid phase, dehydration takes place via the nucleation and spreading of etch-pit like pattern. Laterally, the thermal etch pits spread in an unrestricted way. In the vertical direction, pit growth was limited to a few micrometers. Dehydration by monolayer pits and nucleation of the dehydration process at monolayer steps on the (010) surface were never observed. Thus, unlike growth or dissolution, surface energy related to kink sites or individual point defects seems to be insufficient to trigger dehydration. The temperature-dependent lateral pit growth yields an activation energy of 119 ± 11 kJ/mol. The product phase disintegrates at the parental surface into nano-size particles without any morphologically noticeable transition zone.

The carbonatation of gypsum: Pathways and pseudomorph formation

Abstract In this paper, we present an experimental study of the interaction between gypsum (010) surfaces and aqueous solutions of Na2CO3 with different concentrations. This interaction leads to the carbonatation (i.e., the transformation into carbonate minerals) of gypsum crystals, which under ambient conditions shows the characteristics of a mineral replacement and leads to the formation of pseudomorphs consisting of an aggregate of calcite crystals. Carbonatation progress was monitored by scanning electron microscopy (SEM) and glancing incidence X-ray diffraction (GIXRD). The carbonatation advances from outside to inside the gypsum crystal and occurs through a sequence of reactions, which involves the dissolution of gypsum and the simultaneous crystallization of different polymorphs of CaCO3 [amorphous calcium carbonate (ACC), vaterite, aragonite, and calcite], as well as several solvent-mediated transformations between these polymorphs. The sequence in which CaCO3 phases form is interpreted taking into consideration nucleation kinetics and the qualitative evolution of several chemical parameters in the aqueous solution. The textural characteristics of the transformed regions are described. The degree of faithfulness of the pseudomorphs obtained is related to the kinetics of the carbonatation process, which in turn depends on the initial concentration of carbonate in the aqueous solutions. Finally, changes in the rate at which the transformation front advances are discussed on the basis of both textural and physicochemical considerations.

Nucleation of solid solutions crystallizing from aqueous solutions.

The study of nucleation and growth mechanisms of salts from aqueous solutions, as a function of supersaturation, is described using both macroscopic and microscopic experiments. In situ observations in a fluid cell in an atomic force microscope (AFM) reveal phenomena not accounted for in standard crystal–growth theories, specifically on the role of the crystal structure of the substrate in controlling spiral growth and two–dimensional nucleation. As a model example, the crystallization of two isostructural salts, BaSO4 and SrSO4, is described. The growth of solid–solution crystals is considerably more complex. The supersaturation of a given aqueous solution relative to a solid solution is different with respect to each solid composition, and it leads to the possibility that different compositions can simultaneously grow by different mechanisms on the same crystal face. Oscillatory compositional zoning is another consequence of the interplay between the thermodynamics and the kinetics of nucleation. The factors which control nucleation and growth of the solid solution (Ba,Sr)SO4 from an aqueous solution are described. The predictions made from the theory are compared with direct observations of crystal growth in an AFM.

Comment: Supersaturation in binary solid solution-Aqueous solution systems (Comment on “Crystallization kinetics in binary solid solution –aqueous solution systems” by Alexander G. Shtukenberg, Yurii O. Punin and Pavel Azimov, American Journal of Science, v. 306, p. 553–574.)

Shtukenberg and others (2006) present a general model for predicting growth rates in solid solution – aqueous solution (SS–AS) systems. The key parameter for such a model is the supersaturation, which is particularly stressed and discussed throughout the text. The paper starts with a review of the equilibrium thermodynamics in SS-AS systems on the basis of the Lippmann “total solubility product” (Lippmann, 1980; Glynn and Reardon, 1990; Glynn and others, 1990; Glynn, 2000). Then, the authors discuss the so-called “quasi-equilibria”, defined as those states where “the driving force for the ionic fluxes of one or both components between aqueous solution and solid solution can be negligible”. Finally, the approach is completed by considering the misfit strain arising at the interface between the crystal surface and a newly deposited solid-solution layer of different composition. According to Shtukenberg and coworkers, this strain is expected to have a strong effect on the physical-chemistry of crystal growth, and their model promises to predict the growth and dissolution behavior of solid-solution crystals from aqueous solutions. Whereas the proposed model contains interesting ideas and most of the derived equations are formally correct, some conclusions and, particularly, the underlying terminology are misleading and may result in a wrong interpretation of SS-AS physical-chemistry. There are two main parts to this discussion. The first disputes the concept of “quasi-equilibrium” as defined by Shtukenberg and others (2006) and the second revises the expressions of supersaturation applied to these kinds of systems. In addition, we introduce some remarks on other aspects of the model that are directly or indirectly affected by the concept of quasi-equilibrium and supersaturation of Shtukenberg and co-workers. The physical-chemistry of SS-AS systems is a complex issue that has generated a number of controversies during the last three decades (Thorstenson and Plummer, 1977; Lafont, 1978; Garrels and Wollast, 1978; Thorstenson and Plummer, 1978; Stoessell, 1992; Glynn and others, 1992; Königsberger and Gamsjäger, 1992; Glynn and Reardon, 1992). At present (Glynn, 2000), most of those controversies can be considered as ended, but many kinetic aspects remain unsolved and it is therefore not unusual that new controversies arise. In our opinion, one of the key factors to progress in the understanding of the growth/dissolution behavior in SS-AS systems is to avoid the use of misleading concepts. This is the approach that we mean to give to the present criticism.

Microscopic and spectroscopic investigation of the calcite surface interacted with Hg(II) in aqueous solutions

Abstract The interaction of the {101̅4} cleavage surface of calcite with Hg(CH3COO)2 aqueous solutions with concentration of 5 mм Hg(II) (pH ≈3.5), was investigated using microscopic and spectroscopic techniques. In situ atomic force microscopy experiments showed that surface microtopography changes significantly as a result of the interaction, and that the initial rhombic etch pits induced by H2O dissolution are rapidly transformed to deeper etch pits exhibiting an unusual triangular shape. The growth of these etch pits is strongly anisotropic, moving faster along the [22̅1] direction than along the [010] direction (with step-retreat velocities of ~12 nm s-1 and ~4 nm s-1, respectively). The modified etch pits are due to Hg(II) sorption in the surface, rather than due to the effect of the acetate anion. The sorption (adsorption and probably absorption also) of Hg(II), in the first minutes of the interaction, is shown by X-ray photoelectron spectroscopy. After ~2 h, the triangular etch pits are interconnected to form larger hexagonal etch pits, while Hg(II)-bearing phases (confirmed later by SEM-EDS) grow onto the surface through a heterogeneous nucleation process. The crystal growth of orthorhombic (montroydite-type) hydrated Hg(II) oxide (HgO·nH2O) on the surface of calcite was confirmed by XRD patterns and FT-IR spectra from samples exposed for longer times to Hg(CH3COO)2 solution.

Nucleation and Growth of Scheelite in a Diffusing-Reacting System

In this paper a study of the nucleation and growth behaviour of scheelite (CaWO 4 ) in a porous medium at 25°C is presented. The experimental set-up involves the counter diffusion of calcium and tungsten through a silica gel column. This results in the continuous evolution of pH and Ca 2+ and WO 4 2- concentrations and therefore in the generation of scheelite supersaturation profiles along the gel column. The limited ionic mobility of the system determines that nucleation occurs at high supersaturation. The characteristics of scheelite nucleation (waiting time, precipitate position and metastability levels) are explained by attending to both the way in which supersaturation is created and the [Ca 2+ ] and [WO 4 2- ] distribution along the diffusion column. Crystal morphologies developed after nucleation are consistent with the high supersaturations observed. The results presented here are in agreement with the general crystallisation behaviour reported by PRIETO et al. (1989, 1991,1997) for other sparingly soluble salts.